Since a metal oxide magnetic powder such as iron oxide and chromium oxide magnetic powder has better magnetic stability than metal or alloy magnetic powder, it is conveniently used as a ferromagnetic powder for magnetic recording mediums. Recently, as the magnetic recording medium is designed to have better properties, attempts have been made on increasing the coercive force of the metal oxide magnetic powder so as to achieve high density recording and simultaneously increasing saturation magnetization of the magnetic powder so as to achieve large output such as high S/N ratio.
Although cobalt-containing iron oxide magnetic powder is modified to have the large coercive force required for high density recording by increasing the amount of cobalt content, its saturation magnetization is at most 80 emu/g, which is not sufficient for large output. The cobalt-containing iron oxide-.magnetic powder includes one in which cobalt and iron oxide form a homogeneous solid solution and one in which a layer containing cobalt oxide is formed on the surface of the iron oxide magnetic powder particle by treating the powder in an alkaline solution containing a cobalt salt.
As one kind of metal oxide magnetic powder having both large coercive force and large saturation magnetization, Japanese Pat. Kokai Publication (unexamined) No. 132005/1976 discloses chromium oxide magnetic powder having a metal cobalt coating formed by electroless plating. Japanese Pat. Publication 24714/1976 discloses magnetic powder produced by dispersing magnetite (Fe.sub.3 O.sub.4) fine particles in an aqueous solution containing cobalt ion and precipitating metal cobalt on the surface of the fine particles by contacting the solution with a hydrogen reducing agent at a high temperature under a high pressure.
Since these types of magnetic powders have such structures that the metal cobalt layer is formed by reduction on the surface of the magnetic powder particles, they are expected to have not only large coercive force but also large saturation magnetization so that they are suitable for a magnetic recording medium with a high density recording and large output. However, in spite of the above characteristics, their squareness ratio (remanent magnetization/saturation magnetization) becomes smaller than that of the untreated magnetic powder, and dispersibility and orientation in a magnetic layer tend to be deteriorated. Thus, it is still desired to improve the properties of the magnetic powder which relate to the high density recording and large output of the magnetic recording medium.